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Net closing on coral reef bombers

Coastguards will soon be hot on the trail of fishermen who are illegally blasting many coral reefs to rubble as they use bombs to increase their catch.

Up come the fish, as more coral turns to rubble

(Image&colon; HKUST)

“Blast fishing is often known to occur in a region through sporadic arrests and anecdotal observations, yet the scale of the problem is often not appreciated as most blasts go undetected,” says George Woodman, who works on the listening project led by the marine sensors group at the Hong Kong University of Science and Technology.

One problem that has hampered development of a detection system for underwater explosions is the cacophony produced by the claw-clicking “pistol” shrimp that live on reefs. Pistol shrimp near the detection system can generate short-range shock waves that are bigger than the signal from a distant bomb. But now the Hong Kong team has solved the problem.

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The researchers use underwater microphones, or hydrophones, to pick up the noise from blasts. The detection range for each hydrophone is around 30 kilometres and the team has developed software to calculate the direction of a blast from the slight time difference between the noise reaching each of three hydrophones. Two such sets of hydrophones could be used to triangulate the position of an explosion to within 30 metres over a 10-kilometre range.

Fertiliser and fuel

Woodman and his team first had to make sure they could reliably recognise the sound generated by home-made bombs in the shallow waters where reef-smashing fishermen operate. They set off controlled explosions using bombs made from a mix of fertiliser and fuel oil – using sandy areas of the seabed to minimise ecological impact.

Their system distinguishes the noise of an underwater explosion from that of the shrimps’ clicking by recognising differences in the energy of the sound produced. An explosion contains more energy overall and lasts longer than the clicks. In contrast, the noise from an outboard motor is more prolonged, but its peak signal is smaller. Their system will be described in a future issue of Marine Pollution Bulletin.

The team has already tested one of their hydrophones in a survey for the Sabah Parks Authority in Tunku Abdul Rahman National Park, off Kota Kinabalu in Sabah, Malaysia.

“Over a 10-day stretch we picked up 15 blasts using one listening station [hydrophone] and we are very confident we can determine the direction of the blasts to within about 0.2 degrees,” says Woodman. They now hope to mount a trial with three hydrophones.